1. Field of the Invention
This invention relates to a facsimile apparatus having a function of converting the reading resolution if necessary to transmit data at an optimum resolution.
2. Description of the Related Art
Transmission resolution is included in the parameters which are to be determined in advance by the operator for facsimile transmission using this kind of apparatus.
For example, in a case where an original is read before calling to enable transmission from a memory, it is read while the resolution of the receiver on the other end of the line is unknown; it is ordinarily read at a resolution suitably determined by the operator and is stored in the memory. The resolution is selected by the operator according to the contents of the original to be transmitted. Ordinarily, a high resolution is selected if the contents of the original include small characters and fine drawings, or a low resolution or a low-transmission-cost mode is selected if the contents consist of comparatively large characters.
However, there is a possibility of inconformity of the resolution selected on the transmiting side with the receiving resolution of the receiver on the other end. Actually, in such a case, communication is effected at a standard resolution provided in any facsimile apparatus. The standard resolution is defined as a main scanning.times.sub-scanning resolution, 200.times.200 ppi in the case of G4 facsimile (ppi: pels/inch) or 8 pels/mm.times.3.85 line/mm in the case of G3 facsimile.
In a case where a facsimile apparatus having two transmission resolutions of 400 and 200 ppi is used for transmission and where the receiver has two receiving resolutions of 300 and 200 ppi, the operator may select the transmission resolution of 400 ppi. Under this condition, communication at 200 ppi is performed, which result is contrary to the operator's intention of performing high-resolution communication.
Again, in a case where a facsimile apparatus having two transmission resolutions of 300 and 200 ppi is used for transmission and where the receiver has two receiving resolutions of 400 and 200 ppi, the operator may select the transmission resolution of 300 ppi. Under this condition, communication at 200 ppi is performed, which does not meet the operator's intention of performing high-resolution communication.
Ordinarily, the reader of the facsimile apparatus for reading images is designed so as to read images at the highest transmission resolution of the apparatus. This resolution ordinarily coincides with the recording resolution of the recording unit.
In the above cases, the image read at 400 or 300 ppi is transmitted by converting the resolution into 200 ppi. A similar procedure is also used in the case of multicast communication of image data stored in a memory. That is, suitable resolutions of transmitters for other-side terminals are selected and an image read at a high resolution is converted so that its resolution changes into the selected resolutions. Needless to say, the available resolutions are limited within the range of nominal resolutions of the apparatus on the transmitting side.
Conventionally, such resolution conversion is performed by simple image processing such as thinning of pixels or OR processing. The conventional methods using such techniques entail the problem of deteriorations in image qualities and have considerable drawbacks in terms of practice with respect to half-tone images in particular. In some case, the image qualities of the received image are seriously deteriorated depending upon the combination of the resolutions of the transmitter and the receiver. In the case of a facsimile apparatus having both the functions of group 4 and group 3 apparatuses (hereinafter referred to as "G4 apparatus" and "G3 apparatus", respectively) specified by CCITT (International Consultative Committee for Telephone and Telegraph), a need for converting the resolution from the inch system for G4 apparatuses to the millimeter system for G3 apparatuses arises when, for example, an image read at the G4 reading resolution is transmitted to a G3 apparatus. The same problem as that mentioned above is thereby encountered.